An artificial hip joint usually comprises a joint socket provided with an insert into which the head of a shank engages. Analogous to the natural joint to be replaced, the artificial joint socket must be mounted in the socket of the pelvic bone of the patient in a positionally accurate and stable manner. Various artificial hip sockets are known, with two types of socket being most commonly used: conically shaped sockets and spherical sockets. The decision to use one or the other socket type is dependant on the medical condition and the preference of the operating physician.
Both socket types comprise a metallic outer shell, an insert, and an inlay arranged in the outer shell. The inlay is usually made of ceramic, plastics, such as polyethylene (chirulene), or metal. Such an artificial joint socket, having a shell and an insert, has been shown to be very reliable.
One advantage of using the spherical sockets is that during pre-milling of the spherical bed, less pelvic bone needs to be removed than with a conically-shaped socket, since the acetabulum is approximately spherical even with greatly deformed joints. The milling process is also easier with spherical sockets than with conical sockets since milling does not need to be so accurate. Further, the danger of the ileum, ischium or pubis becoming damaged due to milling too deep is much lower with a spherical bed than with a conical one.
In addition to the standard design of a spherical socket which corresponds to a hemisphere, press-fit designs have also been successfully applied. The press-fit sockets are slightly flattened at the pole and are somewhat larger in diameter than the pre-milled hemispherical bed in the bone. The force-fit of the socket in the pre-milled bone is thus ensured. A rough surface coating is provided on shells made of metals, such as pure titanium, to permit the intergrowth of the bone cells onto the implant, ensuring an optimal secondary stabilization in the long term. One may also fill behind the press-fit socket with spongiosa in order to achieve an improved retention on the bone. The primary stability of a press-fit socket is usually increased by several additional spongiosa screws or other fixation means. As described in EP-B-0,601,224 and EP-A-0,943,304, the primary stability of a screw socket is increased by way of self-cutting threads which run around the outside of the socket base body.
In contrast to screw sockets, spherical sockets without threads, and press-fit sockets are pressed into the pre-milled bed in a linear movement.
Standardized socket sizes of 44 to 66,i.e. sockets with a diameter of 44 to 66 mm on the socket base surface, are common and readily available on the market.
Various devices have been designed in order to achieve an improved primary stability with these socket types. An artificial joint socket with a basal flange is described in U.S. Pat. No. 4,173,797 in the year 1979, in which, in the inserted condition, the socket bears on the surface of the bone and as a result, prevents tilting of the outer shell. Additional barbs in the pole region of the convex outer side of the shell are also described. The barbs act as rotational sacraments before the implant grows in. U.S. Pat. No. 5,972,032 describes another type of spherical implant having barb-like projections, or spikes, in the pole region. The press-fit socket shown in this reference is knocked in essentially along the longitudinal or rotation axis of the shell body, and the socket is fixed in a rotationally secure manner in the knocked-in condition by way of the spikes, which are arranged essentially parallel to the longitudinal or rotation axis of the implant in the pole region. These spikes, however, only slightly contribute to safeguarding against tensile loads. It is known that compressive forces with a significant axial component in the femur may be built up with press-fit sockets. Moreover, the above described spikes may not primarily safeguard against these forces. Two additional solutions to the problem of insufficient primary securement are described in U.S. Pat. No. 5,755,799 and U.S. Pat. No. 6,231,612. In order to permit a screwless and cementless fixation in the bed of the bone, both implants in the equatorial region of the outer side of the shell comprise a multitude of small projections. The tips of these scale-like or tile-like barbs point distally with respect to the body of the patient, and are thus counter to the knock-in direction. On account of manufacturing technology, the tips follow the contour of the superficies exactly, and the individual rows of scales run from the shell opening in the direction of the pole parallel to the knock-in direction. The manufacture of such implants is very complicated and is thus very expensive. In addition, the barbs are only a few tenths of a millimeter high, and often do not provide the freshly inserted implant with the required amount of primary stability.